Differential response of planktonic primary, bacterial, and dimethylsulfide production rates to static vs. dynamic light exposure in upper mixed-layer summer sea waters
Microbial plankton experience short-term fluctuations in total solar irradiance and in its spectral composition as they are vertically moved by turbulence in the oceanic upper mixed layer (UML). The fact that the light exposure is not static but dynamic may have important consequences for biogeochemical processes and ocean–atmosphere fluxes. However, most biogeochemical processes other than primary production, like bacterial production or dimethylsulfide (DMS) production, are seldom measured in sunlight and even less often in dynamic light fields. We conducted four experiments in oligotrophic summer stratified Mediterranean waters, where a sample from the UML was incubated in ultraviolet (UV)-transparent bottles at three fixed depths within the UML and on a vertically moving basket across the same depth range. We assessed the response of the phyto- and bacterioplankton community with physiological indicators based on flow cytometry singe-cell measurements, fast repetition rate fluorometry (FRRf), phytoplankton pigment concentrations and particulate light absorption. Dynamic light exposure caused a subtle disruption of the photoinhibition and photoacclimation processes associated with ultraviolet radiation (UVR), which slightly alleviated bacterial photoinhibition but did not favor primary production. Gross DMS production (GP DMS) decreased sharply with depth in parallel to shortwave UVR, and displayed a dose-dependent response that mixing did not significantly disrupt. To our knowledge, we provide the first measurements of GP DMS under in situ UV-inclusive optical conditions.